Use nicer Makefile

With this patch, it can be installed with commands like `install PREFIX=/usr DESTDIR="/home/pkgdir"`, which is kinda required to make packages. I made those change so as to create a package for archlinux.

I'm no good with Makefile, I somehow took ideas from an example found on the net (<http://mirror.hmc.edu/ctan/support/highlight/makefile>)